In the past, there has been proposed a remote monitoring system which includes a plurality of monitoring terminals (slave stations) configured to monitor an electric device and generate a monitoring data indicative of monitoring information of the monitored electric device, and a data collection device (master station) configured to collect the monitoring data from the plurality of the monitoring terminals (see Japanese patent laid-open publication No. 2007-295163).
In the conventional remote monitoring system, the plurality of the monitoring terminals is connected to the data collection device via a two wire system communication cable. The data collection device is configured to poll the monitoring terminals to collect the monitoring data from the monitoring terminals. That is, the data collection device transmits the polling signal to the plural monitoring terminals in sequence. Upon receiving the polling signal, the respective monitoring terminals transmit a response signal including the own monitoring data to the data collection device. Consequently, the data collection device collects the monitoring data from all the monitoring terminals.
RS 485 is relatively common as a communication method between the data collection device and the monitoring terminals. However, RS 485 has the maximum connectable number of 32. Therefore, if the number of the monitoring terminals exceeds 32, the remote monitoring system needs a plurality of the data collection devices. In this situation, the remote monitoring system needs an additional device which collects the monitoring data from the plurality of the data collection devices.